Microbes are versatile organisms that have the ability to adapt to a myriad of environments in which they must subsist. Pathogenic bacteria, such as Escherichia coli are no exception to this rule. Its phenotypic characteristics in in vitro medium such as LB broth are not equivalent to its phenotype when it causes a bacteremia. These changes in vivo undoubtedly play an important role in pathogenesis. We are therefore interested in identifying the genes responsible for these in vivo phenotypic changes and their mode of induction. Initial studies suggest that the combination of serum plus hemoglobin can induce these gene products. To identify these genes we will use either transcriptional fusions or create a subtraction library. Once the genes of interest are identified their protein products can be elucidated.